1. Field of the Invention
The invention relates in general to respiratory face masks and more particularly to methods and apparatus that are especially useful for determining the degree of air-tight fit of a mask worn on the face of a user.
2. Description of the Related Art
Respirators, also known as face masks or gas masks, are used to protect personnel from breathing in contaminants while exposed to a contaminated environment. Respirators fall into two basic classes, the first class being a supplied air respirator in which a flexible hose connects a supply of clean air to the respirator, and the second class where the respirator draws air from a surrounding contaminated environment. The latter class is the most widely used of all respirators and respirators of this class generally are constructed to cover the wearer's nose and mouth with a flexible rubber mask which is held in place with an air tight relationship to the face as much as possible through the use of one or more elastic holding straps that encircle the wearer's head.
Respirators typically include a face piece (the part which covers the nose and mouth of the wearer) that may be constructed of rubber or silicone rubber. The face piece is held in place by means of the aforementioned rubber or elastic head bands which usually attach, by means of snaps, to the face piece and surrounds the head in one or more loops.
In the typical respirator of the second class, three apertures are formed in the face piece, two on opposite sides and one in the lower center area (see FIG. 1). The two apertures on opposite sides are designed to receive the inhalation filter cartridges which are the means by which contaminants are filtered from the environmental air and provide the path for air pulled into the face piece by the negative pressure created interiorly by the person inhaling. These inhalation filter cartridges, which appear to be extensions of the wearer's cheeks, are built-up devices having cartridge adaptors, inhalation valve flaps, filters of different types, perforated filter covers, gaskets, and the like. In addition, interchangeable cartridges are available that combine the filter and filter cover into a single cartridge which is screwed on to threads formed on the cartridge adaptor. The cartridge adaptor is in an air-sealed relationship to the face piece. In the lower center portion of the face piece is the exhalation valve, which opens during the time the wearer is exhaling, i.e., when there is an over-pressure interiorly to the face piece relative to the environment, and the exhalation valve closes when the wearer inhales, i.e., there is a negative pressure interiorly to the face piece relative to the environment. In addition, it is common also to place oppositely operating but similar type valves in the inhalation filter cartridges, i.e., upon an over-pressure interiorly to the face piece, the valve closes.
By interchanging different types of filter elements, a respirator may be specifically designed for a particular environment. For example, activated charcoal acts as a scrubber for gases whereas felt, cloth, or paper may be utilized in a paint aerosol environment.
As can well be imagined, of primary concern is the fit of the respirator against the face of the wearer insomuch as, if there is not an air tight fit, the environment will be drawn into the face mask upon inhalation, thus at least partially defeating the purpose of the respirator. Various tests and methods have been devised to determine a “fit factor” for a respirator as applied to a certain person, and the way the test is designed, the higher the number the better the fit. Thus, as defined in the art, the fit factor is a ratio of the contamination level outside the mask divided by the contamination level inside the mask; or alternatively the ratio of total (purified+contaminated) air inspired divided by contaminated air inspired. For example, if a person breathes in air at a rate of 35 liters/minute and it has been determined that 350 milliliters/minute did not enter through the purifying inhalation filter cartridges, the fit factor is a ratio of 35 l./minute ÷0.35 l./minute=100.
The most common method used today of determining the fit factor for respirators is to place a person in an environment with a known concentration of contamination, collect air from the mask interior, and then determine the concentration of the contaminant in such collected air. Air borne contaminants which are commonly used in tests of these types include: di-octal phthalate, commonly called DOP, corn oil, sodium chloride salt fogs, and ambient aerosols. The techniques by which monodispersed contaminant particles are precisely generated and uniformly dispersed in air for these tests are generally rather complicated.
Another major problem in evaluating respirators through today's methods is how the concentration of the air borne contaminant, more commonly called aerosols, is measured. One of the most popular methods used today is to measure concentration through light scattering techniques, i.e., shining a light through a known volume of the captured contaminants and then determining concentration through photometric cell measurement of scattered light.
However, this method has problems in many cases. First, the measuring equipment usually lies some distance away from the party under test (usually outside a sealed chamber) and hoses used to convey the breathed air with contaminants may be porous or partially porous to the particular contaminant or may adsorb the contaminant. Second, as may well be imagined, since wearers' faces are differently shaped and sized, one respirator is not going to fit all people. Accordingly, companies manufacture different sizes. Nevertheless, from the very fact that there are different sizes available in most respirators, attempts to fit the respirator to one particular person mean that there is still a compromise. In addition, the rate of contaminant leakage changes as the wearer breathes at different rates and volumes due to the strenuousness of the wearer's activity. Thus, the fit factor determined for a wearer in a resting condition may not adequately describe the fit factor achieved with the same respirator under more vigorous work conditions.
Consequently, missing from the field of respirator fit data is how well respirators fit a person and what degree of protection is afforded a wearer who wears the mask over a long period of time and under varying conditions of work.
During inhalation, or, as more commonly called in the field, “inspiration”, the inspiratory volume and the inspiratory flow rate, i.e., the rate of movement of air into the wearer's lungs, causes a negative pressure difference between the environment outside the mask and the interior of the face mask. Increasing inspiratory volume and increasing inspiratory flow rate causes a greater negative pressure to be induced inside the mask during more rigorous work conditions. The varying of negative pressure interiorly to a mask simulates varying conditions of work of the wearer, and thus provides a method for determination of fit factor under the varying conditions.
In addition, because of the time, expense, and difficulty in determining a fit factor for a particular respirator, many workers who wear respirators day in and day out are never checked to see which respirator, of all available respirators, achieves for them the highest, and thus the safest, fit factor in order that maximum protection may be afforded.
One approach to the problems encountered with respirator-fit testing is disclosed in U.S. Pat. No. 4,765,325. This patent discloses a system and a method for determining face respirator fit by measurement of leakage air into the interior of the respirator. The method generally included the steps of sealing the respirator against the inhalation and exhalation of air; placing the respirator on the face of the user; having the user inhale air and hold his breath; achieving a desired vacuum within the respirator by evacuating air therefrom; monitoring the pressure interiorly to the respirator; withdrawing air from the respirator to maintain constant the desired vacuum; and measuring the air withdrawn from the respirator, whereby knowing the air withdrawn to maintain the constant partial vacuum air pressure, the leakage air is known and the fit of the respirator determined.
While the invention above advanced the state of the art, experience has shown that the improper sequencing of the test steps, or failure of the subject to comply with test requirements, can have adverse effects on test quality and results. For example, if a test subject prematurely closes the breath inhalation valve of the mask before completing the “preparatory” inhalation that precedes the “holding breath step,” a substantial amount of negative pressure can be trapped inside the respirator, thereby disrupting the remaining test steps. Experience has also shown that the existing test apparatus is very sensitive to any volumetric and pressure changes associated with the test subject's head or facial movement. Often such movement will require that a test be repeated. Finally, previous test protocols involve at least two persons—the test subject and the test administrator. Sometimes a test subject becomes “fidgety” or even fearful during a test because someone else is controlling the progression of the test (and hence the amount of time that the respirator is sealed and the wearer's breath must be held). Such problems have led some evaluators of the prior controlled negative pressure testing method to doubt the veracity and/or general usability of controlled negative pressure fit testing.
Accordingly, it is apparent that there exists a need for new and improved methods and apparatus by which the fit factor for any one mask upon an individual's face may be determined while, preferably, the test subject has control over the test and can perform the testing method under conditions which he or she may expect to encounter during the work day.